In high accuracy servo systems requiring fast settling times and precision positioning any vibrations within the frequency range of the system's required velocity loop bandwidth create a major control system problem with stability. If these conditions are also highly nonlinear or multivariable dependent, the problem becomes even worse.
In a high accuracy positioning system using a variable reluctance linear motor, these vibration problems occur mainly at the point when the system is moving slowly or approaching the desired location. One cause of these vibrations is the high normal forces associated with the stator and armature of the motor, and the interaction between the stator, armature, bearings, commutation, feedback mechanism, etc., which cause nonlinear vibrations to excite resonance frequencies.
When such a motor is operating at high accelerations, the currents applied to the motor phases are relatively high. These high currents generate a strong magnetic attraction (normal) force between opposing motor cores. The strong normal force helps to preload the mechanical pieces of the motor and helps eliminate any hysteriesis or "slop" within the system. As the motor positions the load near its desired location, however, the currents within the motor phases are reduced, thereby reducing the normal force and relaxing the mechanical system. This effective reduction in stiffness adversely affects the velocity loop stability allowing undesirable resonances to occur.
One way to eliminate or control the vibration would be either to lower the velocity loop bandwidth or increase the friction in the system. These proposed solutions would, however, have negative effects on system performance in the way of settle time, accuracy and temperature rise. Another method would be the use of low order linear filters such as low pass or notch filters. These however, would have significant effects on velocity loop bandwidth since the vibration/resonance frequency is within the desired system bandwidth.